For economical reasons, the trend in paper industry is to increase the proportion of filler in paper products and thereby to reduce the use of fibres. In addition to low price and good availability, fillers also increase the printability and optical properties of paper. However, a problem related to the increasing of the filler proportion is that the filler addition leads to a deterioration in the mechanical properties of the paper product. These mechanical properties of paper depend on inter-fibre bonding, and fillers inhibit partly this inter-fibre bonds formation due to their rigidity and poor capability of hydrogen bond formation. Increasing the binding between fibres and fillers is thus essential to improve the strength of filled paper. Furthermore, better affinity between the fibres and the fillers will also lead to a better retention of fillers.
The interactions between fibres and fillers have been widely studied, and many different solutions for improving the inter-fibre bonding have been presented. The loss of the paper strength has been reduced, among other things, by using thinner filler particles. Another solution to this problem is to add starch into the fibre suspension, because the adsorption of starch on fibres increases paper strength by increasing the strength of inter-fibre bonds. Although starch is very cost-effective, it cannot be used in high concentrations because of the problems of significant sticky behaviour of starch on forming wire. Furthermore, the addition of fines in paper is another effective way to compensate for the strength loss which is caused by the presence of the fillers. However, added fines may induce dewatering problems.
As described above, many different solutions have been presented to improve interactions between fibres and fillers in order to enhance the strength of the filled paper. However, there is still a need for a method that makes it possible to use a high content of the filler so that the strength of the final paper product will not decrease and so that the method will not cause any other unwanted effects on the manufacturing process.